Lyme disease is an infection caused by a type of bacteria called Borrelia burgdorferi. These bacteria spread to humans through the bite of infected black-legged ticks (a.k.a. deer ticks). These are tiny, spider-like creatures that can be found in wooded, grassy, or brushy areas of the northeastern, mid-Atlantic, and north-central United States, in Europe and Asia. Not all tick bites lead to Lyme disease—only those carrying the bacteria do. The disease got its name from Lyme, Connecticut because this is where it was first identified in the 1970s.